Various types of energy storing systems are known. One type is based on flywheels, that is to say on at least one mass set in rotation by input of energy, which will continue to rotate, under inertia, after the energy input has ceased. The rotating mass is connected to a motor which constitutes a means of inputting energy during the energy-storage periods, or a generator during energy-restoring periods. The heavier the flywheel and the more able it is to rotate quickly with the lowest possible friction, the greater the amount of energy that can be stored. Mounting of the flywheel bearings is therefore of key importance.
In some types of flywheel the bearings are partially relieved of the weight of the flywheel by applying an electromagnetic force.
Another type of flywheel is described in PCT application PCT/NL2009/000248. This flywheel based inertial energy accumulation device comprises a frame and at least one flywheel mounted so that it can rotate relative to the frame about an axis of rotation, as well as means for exposing at least one face of the flywheel to a gas pressure which, by comparison with the pressure applied to a substantially opposite face of the flywheel, generates an upward differential pressure force that at least partially compensates for the weight of the flywheel, such as by means of so-called gas flow slowing means (referred to as a seal in the present application) surrounding the face of the flywheel that is exposed to the gas pressure. It is stated that in this device not only the flywheel bearings are relieved at least partially of the weight of the flywheel, thus increasing their life, but the cost per kWh is also greatly reduced. These gas flow slowing means make it possible to create a drop in pressure head in the leakage space. They are typically formed between the flywheel and a surface integral with the frame. In an embodiment these flow slowing means comprise a labyrinth seal. In such a seal, the gas flow path comprises a succession of special features that generate drops in pressure head (“head drops”). For example, the cross section for the passage of the gas is alternately reduced and enlarged.
Now it has appeared that in a pilot scale model of a device according to said PCT application the total energy losses are unexpectedly high and as a result the time period that the flywheel can deliver energy, e.g. for charging one or more other appliances is relatively short. Therefore the device cannot be operated in an economically feasible manner. Furthermore manufacturing such a device has been complicated because inevitable tolerances in the construction are larger than the design characteristics.
In WO 2011/155838 A1 it has been proposed to use a rotor having an inverted frustoconical shape with a matching (semi-) labyrinth seal in the gap between the upstanding walls. Such a design has a self-centring effect. However, stability and safety of this device leave something to be desired. In general, the vulnerability of the seal is high. In case of emergency, for example failure, defect, accident and the like serious damage to the precisely, often tailor made, special features of the seal is most likely to occur because of (friction) contact between the rotor and the seal. The rotor itself may also become damaged. Furthermore, operation is complex, in particular starting up and maintaining the appropriate gap size.